This invention pertains generally to the field of optical measurements and measuring devices, and more particularly to reflectometers for measuring the characteristics of optical systems.
The traditional method for making reflectometer measurements is known as optical time domain reflectometry (OTDR). This is an important nondestructive technique for analyzing and diagnosing optical systems, and it is particularly useful in the manufacturing, installation and maintenance of optical fiber systems. Briefly, the method consists of injecting a short intense pulse of light into a fiber and measuring the time-dependent backscattering light signal. This measured signal contains information about the location, type and magnitude of discontinuities, defects, and anomalies of the fiber and other factors which affect the light propagation such as temperature, mechanical strain, and electric and magnetic fields. The technique is essentially analogous to radar and sonar ranging techniques, but implemented for electromagnetic radiation at optical frequencies. The value of this technique lies largely in the fact that an optical system can be studied from a single access point, e.g. the input end of an optical fiber. This advantage obviates the necessity for dismantling the system, and it becomes extremely important when one is diagnosing very large systems such as long optical fibers. A review of this technology has been written by Peter Healey in the article entitled "Review of Long Wavelength Single-Mode Optical Fiber Reflectometry Techniques", published in the Journal of Lightwave Technology, Vol. LT-3, No. 4, August 1985, pp. 876-886.
The conventional OTDR technique becomes less useful when it is applied to smaller optical systems because of the limits on the measurement resolution inherent in this technique. The resolution is defined by the length of the injected light pulse. A reduction in this pulse length implies either a reduction in average pulse energy or an increase in peak power, both of which are constrained within certain limits. Furthermore, shorter pulses require greater bandwidths of the pulses, which are also limited for any given system. Typically the resolution obtained with an OTDR measurement is of the order of 10 meters, and in practice, the resolution limit of this technique is approximately 1 meter ["OFDR Diagnostics for Fibre and Integrated-Optic Systems"; S. A. Kingsley and D. E. N. Davies, Electronics Letters, Vol. 21, No. 10, March 1985, pp. 434-435]. Clearly the conventional OTDR method is not useful in analyzing small systems such as integrated-optic circuits, or for high-resolution fiber-optic sensing such as measuring stresses at short intervals along a fiber.
Improved resolution can be obtained by means of a method termed "optical frequency domain reflectometry (OFDR)", or also commonly referred to as FMCW (frequency modulated continuous wave) reflectometry. This method is described in the above-referenced article by Kingsley and Davies, and in the paper entitled "Optical Frequency Domain Reflectometry in Single-Mode Fiber", written by W. Eickhoff and R. Ulrich, published in Applied Physics Letters 39 (9), Nov. 1, 1981, pp. 693-695. This technique consists of injecting a highly monochromatic beam of light into the optical system, varying the frequency slowly with a time-linear sweep and detecting the backscattered signal. This detection is achieved by the heterodyne method, in that the backscattered signal is mixed coherently with the reference input signal. The beat frequency is measured, and gives the position of a reflection point in the fiber. The amplitude of the beat signal also determines the backscattering factor and attenuation factor for the reflected light. The article by Kingsley and Davies, cited above, reports a resolution of about 3 millimeters obtained by this method and estimates that this can be improved to approximately 1 mm with existing technology.
Clearly the OFDR method offers the capability of improved resolution compared to the conventional OTDR technique. Since the OFDR method is a coherent measurement of interference between a backscattering signal and a reference signal it also offers a greater dynamic range and improved signal-to-noise ratio over a standard OTDR measurement of reflected signal power. Since the OFDR method requires only low optical input signal power, the nonlinear effects of optical transmission in the fiber are reduced. However there are also certain drawbacks to the OFDR technique. Not only does the method require a highly monochromatic source, but it is also sensitive to frequency-sweep nonlinearities, and it is limited by the frequency sweep range.
Heterodyne detection has also been used in OTDR systems with very short pulses to achieve resolutions in the micrometer range. Systems of this type are described in a paper by R. P. Novak, H. H. Gilgen and R. P. Salathe entitled "Investigation of Optical Components in Micrometer Range Using an OTDR System With the Balanced Heterodyne Detection", and a paper by P. Beaud, J. Schutz, W. Hodel, H. P. Weber, H. H. Gilgen and R. P. Salathe, entitled "High Resolutional Optical Time Domain Reflectometry for the Investigation of Integrated Optical Devices", published in the IEEE Journal of Lightwave Technology, Vol. 25 (1989), pp. 755-759. For purposes of clarity this technique may be termed "coherent OTDR". These authors report that by using ultrashort pulses and a coherent detection scheme, the OTDR method can achieve resolutions of about 60 .mu.m in air.
Further improvement in resolution has been obtained by another technique known as "optical coherence-domain reflectometry" (OCDR). This method is described in the following three articles:
(1) "Optical Coherence-Domain Reflectometry: A New Optical Evaluation Technique", by Robert C. Youngquist, Sally Carr, and D. E. N. Davies, Optics Letters, Vol. 12, No. 3, March 1987, pp. 158-160; (2) "New Measurement System for Fault Location in Optical Waveguide Devices Based on an Interferometric Technique", K. Takada, I. Yokohama, K. Chida, and J. Noda, Applied Optics, Vol. 26, No. 9, May 1, 1987, pp. 1603-1606; (3) "Guided-Wave Reflectometry with Micrometer Resolution", B. L. Danielson and C. D. Whittenberg, Applied Optics, Vol. 26, No. 14, Jul. 15, 1987, pp. 2836-2842. The OCDR method differs from the coherent OTDR technique in that instead of a pulsed light source one uses a broadband continuous-wave source with a short coherence length. The source beam enters an interferometer in which one arm has a movable mirror, with the reflected light from this mirror providing a reference beam, and the other arm contains the optical system being tested. The interference signal in the coherently mixed reflected light from the two arms is detected by the usual heterodyne method and yields the desired information about the optical system.
In essence, the OCDR technique replaces the beam pulses by the "coherent domains" in a broadband continuous beam, where a domain is defined as a section of the beam in which the light phases are coherently related. The average size of these sections is termed the "coherence length", 1.sub.c, and is of the order: EQU 1.sub.c .about.c/.DELTA..nu., (1)
where c is the speed of light and .DELTA..nu. is the frequency spread of the light source. [Principles of Optics", 4th Edition, M. Born and E. Wolf, Pergamon Press, New York (1970), Section 7.5.8]. The heterodyne detection of the backscattered "domains" is accomplished by the method of "white light interferometry", in which the beam is split into the two arms of an interferometer, reflected by the adjustable mirror and the backscattering site and coherently recombined. This method utilizes the fact that interference fringes will appear in the recombined beam only when the difference in the optical path length between the two arms is less than the coherence length of the beam, 1.sub.c. The OCDR systems described in references (1) and (3) above make use of this principle, and reference (3) shows interferograms of fiber gaps in test systems obtained by scanning the adjustable mirror and measuring the strength of the recombined signal. Reference (1) also describes a modified method in which the mirror in the reference arm oscillates at a controlled frequency and amplitude, causing a time-varying Doppler shift in the reference signal, and the recombined signal is fed into a filtering circuit to detect the beat frequency signal.
Another variation of this technique is illustrated in reference (2), in which the reference arm mirror is at a fixed position and the difference in optical path lengths in the two arms may exceed the coherence length. The combined signal is then introduced into a second Michelson interferometer with two mirrors, one fixed in position and the other being moveable. This moveable mirror is scanned, and the difference in path length between the arms of the second interferometer compensates for the delay between the backscattered and reference signals at discrete positions of the mirror corresponding to the scattering sites. In practice, an oscillating phase variation at a definite frequency is imposed on the signal from the backscattering site by means of a piezoelectric transducer modulator (PZT) in the fiber leading to this site. The output signal from the Michelson interferometer is fed to a lock-in amplifier, which detects the beat frequency signal arising from both the PZT modulation and the Doppler shift caused by the motion of the scanning mirror. This technique has been used to measure irregularities in glass waveguides with a resolution as short as 15 .mu.m ["Characterization of Silica-Based Waveguides with an Interferometric Optical Time-Domain Reflectometry System Using a 1.3-.mu.m-Wavelength Superluminescent Diode", K. Takada, N. Takato, J. Noda, and Y. Noguchi, Optics Letters, Vol. 14, No. 13, Jul. 1, 1989, pp. 706-708].
In short, the OCDR technique offers the capability of high-resolution measurement of optical systems, together with all of the other advantages of coherent reflectrometry. The optical dynamic range obtainable with this technique can exceed 100 dB on the power logarithmic scale, which implies that refractive index discontinuities of 10.sup.-5 producing reflected light of the order of 1 femtowatt can be detected. The fundamental limitation on the resolution is the coherence length of the light source, which can be reduced to a few microns, with a corresponding increase in source bandwidth.
The OCDR, OFDR, and coherent OTDR techniques all share a common problem arising from the polarization properties of the light beam. This problem is based on the fact that interference between two beams of light can only occur when both beams have the same polarization state. More precisely, the interference signal of two light beams is the incoherent sum of the interference signals from the beam components in two orthogonal polarization states. For example, if one beam is linearly polarized in the horizontal direction and the other beam is linearly polarized in the vertical direction, no interference will occur. Ideally, of course, when the entering beam is split into the two arms of an interferometer, and reflected and coherently recombined, the beam polarization is unchanged. Reference (2), discussed above, includes a polarizer and analyzer mutually aligned at the entrance and exit fibers of the first interferometer to enforce this requirement. In practice, any real fiber will cause a certain amount of distortion of the polarization of the light traveling through it. Furthermore this polarization distortion may be time-dependent. Polarization noise and cross-talk in a fiber may be caused by internal and external perturbations from mechanical, thermal and electromagnetic effects, and can produce fading or reduced visibility of the observed fringes in an interferogram. In addition the signature of a given backscattering site can be complicated by the group delay differences between two polarization eigenmodes in a birefringent fiber.
This problem has been recognized by those skilled in the art and has been mentioned repeatedly in the literature; see for example: "Fading Rates in Coherent OTDR", P. Healey, Electronic Letters, Vol. 20, No. 11, May 24, 1984, pp. 443-444; "Birefringence and Polarization Dispersion Measurements in High-Birefringence Single-Mode Fibers", M. Monerie and F. Alard, Electronics Letters, Vol. 23, p. 198 (1987). In fact, OCDR and interferometric techniques have been widely used to measure polarization distortion in fibers; see for example: "Measurement of Spatial Distribution of Mode Coupling in Birefringent Polarization-Maintaining Fiber with New Detection Scheme", K. Takada, J. Noda and K. Okamoto, Optics Letters, Vol. 11, No. 10, October 1986, pp. 680-682; "Chromatic Dispersion Characterization in Short Single-Mode Fibers by Spectral Scanning of Phase Difference in a Michelson Interferometer", J. Pelayo, J. Paniello, and F. Villuendas, Journal of Lightwave Technology, Vol. 6, No. 12, December 1988, pp. 1861-1865; "Three Ways to Implement Interferencial Techniques: Application to Measurements of Chromatic Dispersion, Birefringence, and Nonlinear Susceptibilities", P.-L. Francois, M. Monerie, C. Vassallo, Y. Durteste, and F. R. Alard, Journal of Lightwave Technology, Vol. 7, No. 3, March 1989, pp. 500-513; "Measurement of Mode Couplings and Extinction Ratios in Polarization-Maintaining Fibers", M. Tsubokawa, T. Higashi, and Y. Sasaki, Journal of Lightwave Technology, Vol. 7, No. 1, January 1989, pp. 45-50.
An extensive discussion of this problem is given in the article: "Polarization-State Control Schemes for Heterodyne or Homodyne Optical Fiber Communications", T. Okoshi, Journal of Lightwave Technology, Vol. LT-3. No. 6, December 1985, pp. 1232-1237. In this paper it is acknowledged that the use of polarization-maintaining fibers is theoretically a complete solution to the problem, but this solution is unsatisfactory for practical reasons. The author mentions two alternative approaches, namely a) the use of a polarization-state control device which matches the reference signal polarization with that of the test signal, and b) the use of a polarization diversity receiver (PDR) in which two orthogonal polarization components of the combined signal are detected separately and added later after an appropriate phase compensation. The author goes on to discuss various schemes for implementing approach a), with no further mention of approach b). Several problems are described that arise in one or more of these polarization-state control schemes, namely:
a) Insertion Loss. The insertion of a polarization control device may attenuate the desired reflectometry signal.
b) Endlessness in Control. The polarization-state distortion of the system may fluctuate over an unknown range, and therefore a control device of limited range may require "resetting" periodically.
c) Temporal Response. The polarization state may fluctuate with unknown rapidity. Manual control of the device may be inadequate. An automatic control feedback scheme may have too long a response time to track these fluctuations accurately.
d) Presence or Absence of Mechanical Fatigue. All schemes that include movement of mechanical components in response to polarization fluctuations suffer from the possibility of mechanical fatigue.
Each of the techniques described in the Okoshi article has at least one of the above drawbacks. The conclusion of this paper is that none of the schemes considered are fully satisfactory, and further efforts are required.
The above Okoshi paper refers only to the use of PDR technology in the context of heterodyne or homodyne optical fiber communications. The PDR is an optical heterodyne receiver that detects signals having an arbitrary polarization state, and is described in the article: "Polarization Independent Coherent Optical Receiver", B. Glance, Journal of Lightwave Technology, Vol. LT-5. No. 2, February 1987, pp. 274-276. Further discussions of the PDR technique are presented in the following articles: "Polarisation-Insensitive Operation of Coherent FSK Transmission System Using Polarisation Diversity", S. Ryu, S. Yamamoto, and K. Mochizuki, Electronics Letters, Vol. 23, No. 25, Dec. 3, 1987, pp. 1382-1384; "First Sea Trial of FSK Heterodyne Optical Transmission System Using Polarisation Diversity", S. Ryu, S. Yamamoto, Y. Namihira, K. Mochizuki and H. Wakabayashi, Electronics Letters, Vol. 24, No. 7, Mar. 31, 1988, pp. 399-400; "New Phase and Polarization-Insensitive Receivers for Coherent Optical Fibre Communication Systems", N. Singh, H. M. Gupta, and V. K. Jain, Optical and Quantum Electronics, Vol. 21, (1989), pp. 343-346; "Adaptive Polarisation Diversity Receiver Configuration for Coherent Optical Fibre Communications", A. D. Kersey, M. J. Marrone, and A. Dandridge, Electronics Letters, Vol. 25, No. 4, Feb. 16, 1989, pp. 275- 277. These articles all discuss the use of PDR techniques for heterodyne reception of optical signals to eliminate the fiber polarization distortion problem. No suggestion is made of this PDR technique as a possible means of dealing with the polarization distortion of optical fibers in coherent reflectometry.
To summarize the technological state of the art, it is known that improved resolution and signal-to-noise ratios in reflectometry systems can be obtained by using coherent detection schemes; that is, optical interferometry systems in which the reflection signal is coherently mixed with a reference signal and the resulting interference signal is detected. Furthermore the optimal coherent detection scheme from the standpoint of resolution is the OCDR, in which the resolution is determined by the coherence length of the light source. This resolution can be made very small by using a broadband source.
It is also known that this coherent detection scheme, like all optical coherent detection schemes, depends on the polarization stability of the optical transmission through the system. Interference between light beams can only occur with signals having the same state of polarization. Changes in the polarization of signals in one arm of the interferometer, or uncorrelated changes in both arms, will degrade the resulting interference signal. These variations in polarization can arise from many causes, both internal and external in nature. Furthermore with respect to external causes, the variations in beam polarization may be time-dependent, arising from fluctuations in environmental conditions.
The polarization stability problem is not confined to optical coherent reflectometry systems. It occurs also in optical communication systems using coherent detection schemes. In this context solutions have been proposed of two general types: direct polarization controllers and polarization diversity receivers. The PDR systems have been designed specifically for this context.
In the reflectometry situation, one can eliminate part of the polarization stability problem by careful design and fabrication of the interferometer. Polarization-maintaining fibers can be used, and the instrument can be encased in a housing to substantially insulate it from environmental perturbations. This is only a partial solution because in operation the instrument must be connected to the device being tested, presumably through optical fibers or other transmission means, and polarization distortion may occur in these external fibers or signal conduits. Furthermore the system under test may itself produce variations in the polarization of the reflectometry signal at reflection or refraction sites or in the optical conduits within the system. These perturbations may be environmental in origin and may fluctuate with time in an essentially uncontrollable manner. Therefore, in an optical coherent reflectometer there is always a polarization instability problem with respect to the optical signals being transmitted and received in the arm of the interferometer that is connected to the device under test.